Conventionally, as a test method of evaluating fatigue characteristics associated with heat build-up in a test piece, e.g., vulcanized rubber, by applying dynamic repeated loads to the test piece, ASTM No. D-623-67 Method A is generally used. As a test machine for this purpose, a Goodrich Flexometer which complies with this standard has widely been used.
FIG. 15 shows the schematic arrangement of the conventional Goodrich flexometer used in the above test, the operation of which will be described hereinbelow. Referring to FIG. 15, a rotary motion of a driving shaft 203 driven by a driving motor 201 through a V-shaped pulley 202 is converted into a vertical motion by an eccentric 204 provided to the driving shaft 203. A test piece 210 formed into the shape of a circular cylinder is sandwiched between an upper anvil 211 and a lower anvil 212. A contact 214 of a thermocouple thermally insulated by an ebonite plate 213 is located at the central portion of the lower anvil 212, as shown in FIG. 16. Lead wires 215 extending from this contact 214 are connected to a temperature measurement device 216 to record the temperature of the test piece 210.
Static and dynamic loads are applied to the test piece 210. The compression load of the static load is applied by a balance. Balance weights 222 and 223 are respectively suspended from the two ends of a lever arm 220 of the balance. A load weight 224 having an adjustable weight is placed on the rear balance weight 223. When the load weight 224 is placed, the lever arm 220 tilts to push up the lower anvil 212 which is fixed to the upper surface of the lever arm 220 through an anvil adjustment screw 218 such that its height can be adjusted in the vertical direction. Hence, the compression load is applied to the test piece 210.
A differential transformer 225 is coupled to the rear portion of the lever arm 220. When the lever arm 220 tilts, the amount of its displacement, i.e., the change amount of compression of the test piece 210 is detected by the differential transformer 225. This detection signal is amplified by a motor control circuit (not shown) and converted into a rotation angle of a reversible motor 227. This rotation angle is decelerated by a gear head 228 and converted into a rotation of a rotating shaft, extending in the lever arm 220 in the longitudinal direction, by a worm gear incorporated in the lever arm 220 through an electromagnetic clutch 229. This rotating shaft rotates a helical gear. When the helical gear rotates, the anvil adjustment screw 218 rotates to move the lower anvil 212 in the vertical direction. As a result, the lever arm 220 is always controlled to be horizontal.
After the static load is applied to the test piece 210 in this manner, the eccentric 204 is rotated by the driving motor 201 to move a connecting rod 240 in the vertical direction, thereby moving in the vertical direction a connecting rod plate 242 coupled to the connecting rod 240 through a connecting rod pin 241. A cross bar holding upper anvil 244 is coupled to the connecting rod plate 242 through driving rods 243. When the upper anvil 211 provided in the lower portion vertically moves upon rotation of the driving motor 201, a compression strain (dynamic load) is repeatedly applied to the test piece 210. The amount of displacement of the upper anvil 211, i.e., the amount of deformation of the test piece 210 can be read by a deformation indicator 246 through an indicator rod 245 extending upward from the cross bar 244.
FIG. 17 shows the principle of the Goodrich flexometer described above. As is apparent from FIG. 15, the lever arm 220 can swing at its center and is supported by a knife edge fulcrum 221. The balance weights 222 and 223 each weighing 24 kg are respectively suspended from the two end portions of the lever arm 220 in order to give inertia to the lever arm 220. Furthermore, the load weight 224 is placed on the left balance weight 223 to apply a static load to the test piece 210 from the lower anvil 212 by a lever operation.
When the test piece 210 is strained by the static load to tilt the lever arm 220 to the left, a test piece support 231 connecting the lower anvil 212 and lever arm 220 is lifted upward by a micrometer screw mechanism 230 through rotation of the screw 218 to restore the horizontal state of the lever arm 220. The upper anvil 211 applies a dynamic load, the strain of which has a constant amplitude, to the test piece 210 with an eccentric mechanism. This dynamic load is received by the inertia of the lever arm 220 through the lower anvil 212.
When the fatigue characteristics are to be evaluated based on the above test machine and test method, two types of tests (1) and (2) described below are generally conducted.
(1) A static initial load is applied to the test piece under predetermined temperature conditions. Furthermore, a sinusoidal vibration having a constant amplitude is applied to the test piece. The temperature of heat build-up and the creep amount of the test piece that change over time are measured.
(2) A static initial load is applied to the test piece under predetermined temperature conditions. Furthermore, a sinusoidal vibration having a constant amplitude is applied to the test piece to promote fatigue. The temperature and time at which blow out occurs in the central portion of the test piece are measured.
When a viscoelastic body causes dynamic fatigue, a physical change occurs in the viscoelastic body. This physical change together with heat build-up makes the interior of the viscoelastic body tacky. Volatile substances in the ingredients and the decomposed substance of the viscoelastic body gasify and accumulate in the viscoelastic body. Then, the interior of the viscoelastic body becomes porous so that the gaseous substances finally make a cavity inside the testpiece. This phenomenon is called blow-out.
In particular, in test (2), when measuring the temperature at which blow-out occurs, the test is conducted while applying a vibration to the test piece. The test is stopped when the temperature of heat build-up in the test piece reaches an anticipated value. Then, the test piece is cut into halves, and the blow-out, i.e., the porous state is observed by the human eye. At this time, if blow-out has not occurred, a test is conducted again under the same conditions by using a next test piece. The test is conducted with a higher temperature of heat build-up than the first test. The test piece is divided and observed. Alternatively, if a large number of pores formed by blow-out are observed, the test is conducted by lowering the temperature of heat build-up inversely. Tests are repeated in this manner until the temperature at which blow-out occurs is determined. Accordingly, numerous tests become necessary.
However, the conventional Goodrich flexometer described above depends on a purely mechanical mechanism that applies a static load (load) to the test piece with a lever arm by the principle of a lever, thereby applying a dynamic deformation to the test piece with an eccentric mechanism. For this reason, the knife edge fulcrum that supports the entire inertia acting on the lever arm and the micrometer screw mechanism that requires high precision as it receives all the static and dynamic loads applied to the test piece tend to wear or be damaged. Then, the operating efficiency of the test machine is degraded or a high maintenance cost is required.
The conventional Goodrich flexometer supports the test piece from below with the inertia of the lever arm. If, however, the lever arm is tilted by the static strain of the test piece to delay anvil adjustment with the micrometer screw mechanism, a tilted load is applied to the test piece, disabling accurate measurement.
The conventional Goodrich flexometer measures the static deformation component by mechanical inertia. However, whether the dynamic component is completely removed is doubtful.
The conventional Goodrich flexometer cannot measure dynamic and static stresses (loads) actually applied to the test piece, and regarding deformation of the test piece, it can measure only the average of the dynamic components. Thus, only a macroscopic superficial result can be obtained.
In the conventional heat build-up/fatigue measuring method, since a flexometer which is a mechanical inertial system is employed, measurement can be performed only under the limited conditions that the static load is constant and the dynamic strain has a constant amplitude; a test can only be conducted under conditions that are far different from actual conditions for use. Furthermore, information that are obtained by measurement are temperature rises of the test piece surface caused by internal heat build-up and deformation of the test piece caused by fatigue. However, in the test process,
(1) static and dynamic stresses cannot be measured, and
(2) regarding deformation, only the average of the dynamic components can be measured.
Therefore, only a macroscopic superficial result can be obtained, and basic data necessary for clarifying the physical mechanism of blow-out, starting with a temperature rise caused by internal heat build-up and reaching destruction, cannot be measured, which is a defect in terms of principle.
In measurement of blow-out, since observation is performed by the human eye by dividing the test piece, the same test must be repeatedly performed from the beginning until blow-out is confirmed by exchanging the test piece. This requires time and labor, not providing a high measurement efficiency.
In the conventional Goodrich flexometer, supply, testing, and discharge of the test piece must all be performed by the person in charge of measurement. When data is to be obtained by using many test pieces, the person in charge of measurement cannot leave the flexometer, which is a very large burden.
Since the temperature of heat build-up is measured on the surface of the test piece, the accurate internal temperature is not clear. When a dynamic load is applied, the entire flexometer vibrates to disable accurate measurement, leading to a decrease in measurement precision.
The present invention has been made in view of the above problems, and has as its object to provide a heat build-up/fatigue measuring method for a viscoelastic body and a hydraulic servo flexometer which, when evaluating fatigue characteristics associated with internal heat buildup of a viscoelastic body, e.g., vulcanized rubber, by applying dynamic repeated loads to the viscoelastic body, can conduct the test by applying static and dynamic loads to the circular cylindrical test piece always perpendicularly to its axial direction by using a hydraulic servo mechanism.
It is another object of the present invention to provide a highly durable hydraulic servo flexometer which can directly measure the strain (displacement) and stress (load) applied to a test piece, thereby enabling highly precise measurement and which has no portion that may wear or be damaged.
It is still another object of the present invention to provide a hydraulic servo flexometer which, for the purpose of measuring basic data necessary for clarifying the physical mechanism, can conduct the test under the following four conditions:
(1) Changes in static and dynamic components of a stress are measured under the test conditions that the static strain is constant and the dynamic strain has a constant amplitude.
(2) Changes in static component of a stress and in dynamic component of a strain are measured under the test conditions that the static strain is constant and the dynamic stress has a constant amplitude.
(3) Changes in static component of a strain and in dynamic component of a stress are measured under the test conditions that the static stress is constant and the dynamic strain has a constant amplitude.
(4) Changes in static and dynamic components of a strain are measured under the test conditions that the static stress is constant and the dynamic stress has a constant amplitude.
It is still another object of the present invention to provide a heat build-up/fatigue measuring method for a viscoelastic body, which can set test conditions that are close to actual conditions for use in order to evaluate the fatigue characteristics, can obtain many pieces of information, and can predict blow-out from these information without dividing the test piece.
It is still another object of the present invention to provide a hydraulic servo flexometer in which transfer, supply, testing, and discharge of the test piece are automated to decrease the burden on the person in charge of measurement, thus improving the measurement efficiency, and in which measurement precision can also be improved.